A flat panel display (FPD) that utilizes an optical film (e.g., polarizer and retardation film) can achieve high-resolution display, and has been widely used as a display device that exhibits excellent performance.
A quarter-wave plate that converts linearly polarized light into circularly polarized light, a half-wave plate that changes the plane of vibration of linearly polarized light by 900, and the like are known. Such a retardation film can achieve accurate conversion of specific monochromatic light so that ¼λ or ½λ retardation occurs.
However, a known retardation film has a problem in that polarized light that passes through is converted into colored polarized light. Specifically, since a material that forms the retardation film has wavelength dispersion with respect to retardation, and a polarization state distribution corresponding to each wavelength occurs with respect to white light that includes different light rays in the visible region, it is impossible to achieve accurate 1/4λ or ½λ retardation over the entire wavelength band.
In order to solve the above problem, various types of wideband retardation films that can achieve uniform retardation with respect to light over a wide wavelength band (i.e., retardation films having reverse wavelength dispersion) have been studied (see Patent Literature 1 to 6, for example).
It has been desired to reduce the thickness of a flat panel display as much as possible along with an improvement in performance and widespread use of mobile information terminals (e.g., mobile personal computers and mobile phones). Therefore, a reduction in thickness of the retardation film has also been desired.
It has been considered that it is most effective to produce a retardation film by applying a polymerizable composition that includes a low-molecular-weight polymerizable compound to a film substrate in order to reduce the thickness of the retardation film. Various low-molecular-weight polymerizable compounds having excellent wavelength dispersion, and various polymerizable compositions using such polymerizable compounds have been developed (see Patent Literature 7 to 24, for example).
However, the low-molecular-weight polymerizable compounds or the polymerizable compositions disclosed in Patent Literature 7 to 24 have a number of problems in that reverse wavelength dispersion may be insufficient, or it may be difficult to apply the low-molecular-weight polymerizable compounds or the polymerizable compositions to a film due to a high melting point that is not suitable for an industrial process, or the temperature range in which liquid crystallinity is obtained may be very narrow, or solubility in a solvent generally used for an industrial process may be low. Moreover, since these low-molecular-weight polymerizable compounds and the like are synthesized by performing a plurality of steps using a synthesis method that utilizes an expensive reagent, the production cost increases.
A novel polymerizable hydrazone compound and a novel polymerizable azine compound have been proposed as a liquid crystal material that may solve these problems (see Patent Literature 25 to 30).
However, when the polymerizable azine compound is applied, dried, subjected to an alignment treatment, and photo-cured using ultraviolet rays to form a polymer film, a change in wavelength dispersion may occur depending on the dose of ultraviolet rays. Specifically, when the polymerizable azine compound is used, reverse wavelength dispersion may be lost (i.e., normal wavelength dispersion may be obtained) when the dose of ultraviolet rays applied to effect curing is increased.